A phantom limb sensation sometimes persists after amputation – the real limb is gone, but the person feels as if the limb remains. They can feel it, and even “move” it. Phantom limbs are one of my favorite examples of the fact that our brains construct an internal model of reality (and our bodies are included in that model), and like all biological processes, that process can go awry. Deprived of sensory and motor feedback from the missing limb, sometimes those bits of the brain (the insular cortex) that give us a sense of ownership and agency over a limb continue to function, manifesting a phantom limb to own and manipulate.

In rare cases, this process can lead to the emergence of a supernumerary phantom limb – a third (or fourth or fifth) arm that the person can feel and manipulate.

Researchers Moseley and Brugger recently published a study involving seven subjects with phantom limbs post-amputation. Their question was whether or not the subjects could be made to move their phantom limbs in anatomically impossible ways. They did this by giving them tasks to imagine with their phantom limbs, tasks that would be quick to accomplish by bending their wrist back in a normally impossible way.

Four of the seven subjects reported successfully completing the task. This was experimentally confirmed by timing the subjects in completing their tasks – which were more quickly completed by using the impossible movements.

What is most interesting is that once the subjects successfully moved their phantom limbs in the new way, their internal image of their phantom limb morphed to accommodate the new movement – including a modified wrist joint over which they had a sense of ownership. This change was further corroborated by the fact that previously simple tasks were made more difficult by the new joint configuration.

In other words – the subjects’ internal model of their phantom limb changed as a result of conscious effort to use the limb in new ways. The subjective ease and difficulty of tasks then conformed to the new anatomical model of the phantom limb. What this likely means is that there is a high degree of plasticity in those brain regions that create our internal model of or body parts and ownership over those body parts. This is consistent with the more basic observation that phantom limbs, and especially supernumerary phantom limbs, can exist in the first place.

There are two apparent applications for this plasticity. The first is as a therapeutic option for those with phantom limbs. Sometimes, the phantom limbs are contorted or contracted in uncomfortable positions. Encouraging patients with painful phantom limbs to imagine a task that will relieve the painful position may be an effective treatment. Other interventions have also been used successfully – such as presenting subjects with a virtual image of their phantom limb, which moves into a more comfortable position, triggering their internal model to do so.

And this example brings up the second application for the apparent plasticity of our internal model of our own bodies – prosthetics and virtual reality. In a “six million dollar man” scenario, where an amputee is given a robotic limb, it is plausible that recipients may be made to feel ownership over their new limb, rather than feeling as if they have something attached to their body. Sensory and motor feedback is helpful – but this study suggests it is not even necessary – imagining may be sufficient.

Further, as the world of virtual reality looms before us, it is plausible that our brains can be easily tricked into feeling as if we occupy the virtual representation of ourselves. We will not just be looking at an our avatar in a virtual world, but feel as if we are the avatar and are in the virtual world.

It remains to be seen how easy this will be to accomplish, and how complete the sensation will be. It may be that some sense of ownership will be possible, but will feel unnatural and disconcerting. Or it may be that most people will require considerable training to functionally occupy their virtual selves. Perhaps only the next generation, who begin the process at a young age, will become truly comfortable in their virtual selves. It may also take some time to understand and tweak the process until it becomes practical and comfortable for most people.